1. Field of the Invention
This invention relates to a method and apparatus for mapping the deformation which occurs on a surface of an object as a result of stressing the object and more particularly to such a method employing the coherent photographic techniques.
2. Prior Art
A knowledge of the deformation patterns which occur in an object when the object is subjected to a stress is useful in areas such as stress analysis, vibration studies, inspection of hidden flaws in structures, material properties evaluation, velocity measurement, thermal measurement, and optical component testing. As employed hereinafter, the term "deformation" refers to the relative displacement of points on the object's surface relative to one another resulting from the imposed stress, as opposed to overall, uniform displacements resulting from the stress.
The simplest and oldest method of determining such surface deformation involved the point-by-point mapping of the object surface before and after imposition of the stress employing any of a variety of measurement techniques ranging from contact surface gaging, through strain gages, to the highly precise optical interference techniques. These methods are inherently extremely slow or yield information relating to relatively few points on the object surface and are generally unsuitable for any form of testing that must be regularly repeated, such as for production inspection use.
The invention of practical holography techniques during the 1960's led to the development of holographic interferometric techniques for mapping the displacement of an entire object surface resulting from an applied stress. The object was illuminated with coherent light and a photographic media was exposed to the interference pattern between unimaged light reflected from the object and a reference beam of coherent light from the illuminating source. In the real time version of the process the photographic media was then developed to form a hologram and the object was then stressed and coherently illuminated and the coherent reconstruction, from the hologram, of the image of the object before testing was superimposed on the object itself. Interference between light reflected from the object in real time and the reconstructed image resulted in an array of interference fringes on the object displaced as a function of the deformation of the object as a result of stressing. In the double exposure version of the technique the media was subjected to two exposures made before and after stressing of the object, and when an image of the object was reconstructed from the resulting hologram using coherent light an array of interference fringes resulting from interference between the two exposures and arrayed as a function of the deformation of the object between the two exposures was visible on the object's surface.
These displacement mapping techniques are disclosed in Grant et al U.S. Pat. No. 3,545,259. They enjoyed a certain degree of commercial success in analyzing subsurface flaws and like imperfections in manufactured objects but its application was limited because of the relatively severe limitations of holography. In the practice of holography the model and the optical apparatus must be aligned with a high degree of precision and the set-up must be highly stabilized and isolated from any possible vibration employing heavy damped tables and the like. The size of an object that can be analyzed is severely limited by the coherence length of the laser and extremely high resolution and low speed photographic medias had to be employed. The process had to be performed in the absence of any ambient light other than that provided by the laser and the light was restricted to a single frequency.
These holographic interferometric techniques do not measure the displacement of points on the object surface relative to one another as a result of the applied stress but rather measure the overall translation displacements due to deformation and rigid body motion. Great difficulty is encountered in distinguishing the interference fringes attributable to various imperfections or non-uniformities in the object from those resulting from overall deflection of the object as a result of stressing. For example, if a rubber tire to be analyzed for subsurface non-uniformities were stressed by modifying the ambient pressure on the tire, the surface under examination would undergo an overall deformation and the surface area immediately over the flaw might undergo a greater deformation. In the reconstruction of the image of the object from the resulting double exposure hologram it would be difficult to separate the fringe families resulting from the anomalous deformation from those resulting from the uniform deformation. While efforts were made to modify the optical set-up between exposures to minimize the fringes resulting from overall deformation, for example see Kersch et al U.S. Pat. No. 3,860,346, this problem was never fully resolved and greatly limited the application of the holographic interferometric technique.
The present invention is broadly directed toward an interferometric technique for producing the same broad type of analysis as double exposure holographic interferometry without the limitations of that prior art technique with respect to the accuracy of set-up, support and the like, and moreover to such a method which eliminates the problem of the fringes representative of anomalous distortion being masked by fringes resulting from a uniform deformation of the object as a result of stress. The method of the present invention also obviates the need for the use of a coherent light source to view the resulting interference fringes.